Avoid calling fwrite() with nullptr (which is undefined behaviour).

Signed-off-by: Stefan Westerfeld <stefan@space.twc.de>

src/rawoutputstream.cc

@@ -88,6 +88,9 @@ RawOutputStream::write_frames (const vector<float>& samples)
 {
   assert (m_state == State::OPEN);
 
+  if (samples.empty())
+    return Error::Code::NONE;
+
   vector<unsigned char> bytes;
   m_raw_converter->to_raw (samples, bytes);
 

src/stdoutwavoutputstream.cc

@@ -132,11 +132,14 @@ StdoutWavOutputStream::open (int n_channels, int sample_rate, int bit_depth, siz
 Error
 StdoutWavOutputStream::write_frames (const vector<float>& samples)
 {
+  if (samples.empty())
+    return Error::Code::NONE;
+
   vector<unsigned char> output_bytes;
 
   m_raw_converter->to_raw (samples, output_bytes);
 
-  fwrite (output_bytes.data(), 1, output_bytes.size(), stdout);
+  fwrite (&output_bytes[0], 1, output_bytes.size(), stdout);
   if (ferror (stdout))
     return Error ("write sample data failed");